This invention relates to apparatus for receiving and executing commands transmitted by means of a modulated carrier signal, and more particularly to apparatus of this type, such as transfer-trip receivers, for performing protection and control functions at substations in electrical power transmission systems.
A transfer-trip receiver is a device used at substations to perform protection functions, for example, to interrupt and reconnect high-voltage power breakers in transmission lines upon command. The commands are sent utilizing a carrier signal which is transmitted over the electrical power transmission lines which extend from one substation to another substation.
Transfer-trip receivers and related communication equipment employ narrow band receivers. Power lines, particularly during fault conditions, are noisy. Some portion of this noise typically will enter the narrow band channel that is used for communications. As a result, receivers may interpret noise as a command to open a breaker, thereby producing unnecessary interruptions in service.
As a result, it has become common to equip receivers with squelch schemes that detect noise and block the output of the receivers when noise is detected. It is known to measure the amplitude of analog signals and, where the amplitude falls outside a predetermined range, interpret such a change in amplitude as noise and accordingly generate a squelch signal. However, it has been found in such squelch schemes that security against erroneous interpretation of noise as commands is inversely related to dependability for detection of actual commands. This follows from the fact that reduction of squelching action under noisy conditions decreases security by increasing the possibility that noise will be interpreted as a command, while assuring that a squelch detection system will not interpret a genuine command as noise.